Using Case To Subtract To Condition

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I have data for moths which are commulative, for example Feb where month is 02, ecul Jan. + Feb. data. I would like to have the data for only Feb = when month '02' Feb = data for '02' - data for '01'. I would like to writ these commands: For Jan there is no problem to write the command but for other months I am not sure how to write the commands for months other than Jan.

,Jan_qty_1 = case
when [stat_month] = '01' then cast(sum(cast([qty_1_mo] as bigint)) as bigint)
else CAST('0' as int)
end

[Code] ....

here is the data:

ust_code district stat_year stat_monthqty_1_mo
24105220070112619
2410522007022200
2410522007024438
2410522007030
24105220070314434
2410522007042295
2410522007040
2410522007050
2410522007050
241052200706454